Self driving machines, e.g. agricultural tractors and harvesting machines, which are provided for off-road use but cover relatively long distances at relatively high speed on public roads on the way to and from the locations of use, are preferably provided with a tyre pressure regulating device, by means of which the tyre pressures of the vehicle wheels can be changed, i.e. increased or lowered as required. While driving on paved roads requires a small contact area of the vehicle tyres, achieved by means of an increased tyre pressure, to achieve a low rolling resistance and good adhesion of the vehicle tyres on the roadway, a tyre pressure that is as low as possible is advantageous for off-road use, e.g. during work on a field or when harvesting wood in a forest, preventing the vehicle wheels from sinking into the soft ground and preventing unwanted compaction of the ground and increasing the traction of the driven vehicle wheels by virtue of the enlarged contact area of the vehicle wheels. Since separate compressed air sources with an adequate compressed air supply are generally unavailable, either in stationary or mobile form, modern off-road machines are increasingly being provided with a tyre pressure regulating device which, to avoid long interruptions in driving or work, are preferably designed in such a way that the tyre pressures of the vehicle wheels can be adjusted while traveling. Modern agricultural tractors, in particular, are often fitted with an air brake system, therefore, in this case, the tyre pressure regulating device can advantageously use the existing compressed air supply device of the brake system as a compressed air source.
DE 31 05 037 C2 describes a tyre pressure regulating device of a two-axle motor vehicle, in which a main connecting line which branches into two axle connecting lines can be connected via a relay valve and a shutoff valve to the filling pressure line, which is connected to a delivery pressure line of the compressed air source via a pressure limiting valve. The relay valve and the shutoff valve are of pressure-controlled design and can be controlled by means of a common pilot control valve. Arranged in each of the axle connecting lines, which each branch into two outer wheel connecting lines, is a respective pressure-controlled relay valve, which can be controlled by means of an associated pilot control valve. Those segments of the outer wheel connecting lines which are arranged on the vehicle chassis or on the vehicle axles and those which are arranged on the vehicle wheels are each connected to one another by single-channel rotary couplings. Arranged at each of the vehicle wheels is a wheel valve designed as a pressure-controlled 2/2-way switching valve, by means of which an inner wheel connecting line leading into the interior of the relevant wheel tyre can alternately be connected to the associated outer wheel connecting line or can be shut off with respect to the latter. The control lines of the wheel valves each branch off from the associated outer wheel connecting line. The filling pressure required to increase or lower the tyre pressure is set at a control valve, which is used to control a pilot control valve associated with a relay valve arranged between the filling pressure line and the main connecting line. Irrespective of whether the tyre pressure is being increased or lowered, the wheel valves open into the outer wheel connecting lines when a defined minimum pressure is reached.
Further tyre pressure regulating devices are known from DE 198 04 249 A1 and WO 2012/084690 A1. DE 10 2004 021 161 B4 describes a rotary coupling or rotary union in a tyre pressure regulating device.